The present invention relates generally to electronic ballasts for powering a high-pressure discharge lamps. More particularly, the present invention relates to an electronic ballast with multi-mode lamp starting circuitry.
An electronic ballast as known in the art for lighting a high pressure discharge lamp such as an HID lamp, typically includes a high starting voltage generated due to a resonant effect of an LC resonant circuit formed of an inductor and a capacitor and applied to start and light the high pressure discharge lamp.
The electronic ballast includes an AC/DC converting circuit for converting an AC power input to a DC power output, a DC/DC converter for generating a rectangular wave voltage by switching an output of the AC/DC converting circuit, a load circuit including the high pressure discharge lamp and coupled to receive the DC/DC converter output, and a control circuit for controlling switching operations of the AC/DC converting circuit and the DC/DC converter.
The control circuit controls the switching operations of the AC/DC converting circuit and the DC/DC converter so as to sequentially perform a resonant ignition phase for starting the high pressure discharge lamp, a warm-up phase for generating glow discharge and preheating lamp electrodes, and a steady-state operation phase for applying the rectangular wave voltage to the high pressure discharge lamp to maintain stable lighting of the high pressure discharge lamp.
That is, at startup and lighting of the lamp, first, by electrically resonating the inductor and the capacitor which form the LC resonant circuit in the resonant ignition phase, a sufficiently high starting voltage necessary for producing electrical breakdown of the discharge lamp is generated and the starting voltage is applied between both ends of the lamp.
Next, in the warm-up phase, by applying a voltage of a frequency which is relatively lower than the frequency in the resonant ignition phase to the lamp, glow discharge is generated to preheat the lamp electrodes. In the subsequent steady-state operation phase, by continuously generating arc discharge, stable lighting of the high pressure discharge lamp is maintained.
In many conventional electronic ballasts, when the lamp fades out in the warm-up phase, only a voltage up to a bus voltage of about 300V as the output of the DC/DC converter can be applied to the lamp as a restarting voltage. Thus, when the discharge lamp is completely extinguished, the lamp disadvantageously fails to start. When the lamp fails to start, the resonant ignition phase and the warm-up phase are repeatedly performed, increasing start-up time.
Because the high starting voltage is applied to the lamp in the resonant ignition phase, when the lamp does not start even if the starting voltage is continuously applied within one second, the discharge vapor pressure in a glass bulb needs to be decreased prior to restart and, after a down time from a few seconds to a few minutes, the high starting voltage is applied again. Therefore, in the electronic ballast as conventionally known in the art, even if the lamp is able to be started, the down time from a few seconds to a few minutes is necessarily provided, thereby delaying startup of the lamp.